Frzb is a secreted protein, initially isolated from purified cartilage extracts, which shares homology to the cysteine rich domain (CRD) of the frizzled family of Wnt receptors. The Wnt proteins are secreted signaling molecules having numerous developmental functions, including skeletal development, as well as dysfunction in oncogenesis. It has been shown that Frzb can bind to and inactivate Wnt activity, leading to speculation for its potential therapeutic use in modifying Wnt induced developmental and oncogenic events. We have demonstrated that Frzb is temporally and spatially expressed during skeletal and craniofacial development. In an attempt to understand the role of Frzb in development we conducted a conditional gene knockout, using the Cre-LoxP recombination system. We generated "floxed" mice, in which the Frzb gene is flanked by LoxP sites, by homologous recombination. These mice were subsequently crossed to a transgenic mouse line expressing Cre recombinase under the control of a ubiquitous promoter to delete Frzb early in development. The null phenotype, however, appears normal. This is most likely due to functional redundancy, as there are now known to be a number of related Frzb-like genes with overlapping expression patterns. Consequently, in order to study the role of these genes during development it will be necessary to carry out double knockouts of different family members. It should still be possible to determine the role of Frzb in limb and face development by crossing the floxed Frzb mice to null mice for other family members once they are available. Subsequently, Frzb can be deleted in a tissue-specific manner by crossing the mice with transgenic mice expressing Cre recombinase under the control of tissue-specific promoters. In order to do this we are generating transgenic mouse lines to express Cre recombinase specifically in the limb and facial mesenchyme. These mice, are currently being tested.